Keep shared boundaries after polygon simplification

[wipfli]

Is it possible to simplify polygon boundaries such that they stay shared between neighboring polygons?

I was playing around with landcover polygons and found that douglas peuker makes boundaries different for neighboring polygons. This creates gaps.

[msbarry]

The first problem is detecting that they are shared between polygons. To do it right we'd the input to be in a format like topojson or at least convey which points of a polygons are shared between more than two polygons so we can avoid removing them during simplification. Absent that though we could have a routine do the work to compute shared edges before simplification - except we'd need access to both geometries when simplifying but planetiler only processes individual geometries in isolation so we'd need to do it in post-process.

Another option would be to limit simplification to only remove points that increase the area of a polygon but not decrease it (so it only produces overlaps but not gaps). Would that work?

[bdon]

The overlapping workaround might not be acceptable if the polygons are displayed with alpha < 1

[wipfli]

The size increase would work for my use-case. Thanks for sharing this idea!

[wipfli]

Here is an example of the defect in see: https://wipfli.github.io/median-filter-landcover/#map=9.79/9.6344/104.8832

[msbarry]

I'd also be curious what the output and file size looks like if you set simplify tolerance to 0 during the process step. That would only simplify geometries by rounding to tile coordinates. That needs to look ok for the "topology-aware simplification in post-process" approach to be a viable option.

[bdon]

Tippecanoe has this functionality with --detect-shared-borders

https://github.com/felt/tippecanoe#attempts-to-improve-shared-polygon-boundaries